This invention relates to electrically conductive or semi-conductive devices having an insulating member including an ethylene, xcex1-olefin, vinyl norbornene elastomeric polymer having a branching index of less than about 0.5 and a metallocene catalyzed ethylene xcex1-olefin copolymer and the compounds made from the elastomeric polymer ethylene-copolymer combination providing elastomeric polymer based members having excellent surface characteristics and dielectric strength.
Typical power cables generally include one or more conductors in a core that is generally surrounded by several layers that can include a first polymeric semi-conducting shield layer, a polymeric insulating layer and a second polymeric semi-conducting shield layer, a metallic tape and a polymeric jacket. A wide variety of polymeric materials have been utilized as electrical insulating and semi-conducting shield materials for power cable and numerous other electrical applications.
In elastomer or elastomer-like polymers often used as one or more of the polymer (insulation) members in power cables, common ethylene, xcex1-olefin, non-conjugated diene elastic polymers materials that have come into wide use usually include ethylene, xcex1-olefin, and a non-conjugated diene selected from the group consisting of 5-ethylidene-2-norbornene (ENB), 1,4-hexadiene (HD), 1,6 octadiene, 5-methyl-1,4 hexadiene, 3,7-dimethyl-1,6-octadiene, and the like. Such polymers can provide a good insulating property for power cables. However, generally selection of one of these elastomeric polymers while bringing certain advantages, also brings some disadvantages as well. For instance, electrical compounds containing some of these polymers usually necessitate slower extrusion rates than might be desirable for optimum output, because surface characteristics of the extrudate in a compound based on these elastomeric polymers will not be as smooth as desired if the extrusion rates are higher. By choosing elastomeric polymers containing 5-vinyl-2-norbornene (VNB), a higher level of extrusion output can be achieved in the compound usually due to a higher level of branching, but some diminution of physical properties (over the less branched material) may result.
Much of the production of insulated electrical devices would see an advantage from higher throughput or extrusion rates (eg, lowering manufacturing costs) but many of the so called conventional elastomeric polymers, especially those based on 1, 4-Hexadiene and ENB, would exhibit a tendency to cure slowly.
There is a commercial need for an elastomeric polymer ethylene alpha-olefin copolymer blend insulating material for electrical devices that can be extruded relatively rapidly, in the substantial absence of surface roughness, having a relatively rapid cure rate, relatively high cure state and relatively low electrical loss. There is also a need for improved long term heat aging and lower cure additives consumption, all of which may reduce the overall manufacturing cost and/or improve quality of the cable insulation.
We have discovered that polymeric insulation for electrical conducting devices, when it includes an ethylene, alpha-olefin, vinyl norbornene elastomeric polymer with a relatively low branching index, indicative of long chain branching, will provide a smooth surface at relatively high extruder speeds, and generally will cure faster to a higher cure state than previously available ethylene, alpha-olefin, non-conjugated diene elastomeric polymers. Additionally, inclusion of a minority component of metallocene catalyzed ethylene alpha-olefin copolymer (hereinafter m-ethylene copolymer) enhances the physical properties of the elastomeric polymer in an electrical insulating compound based on the combination or blend.
According to one embodiment of our invention, an electrically conductive device is provided including (a) an electrically conductive member comprising at least one electrically conductive substrate; and (b) at least one electrically insulating member in proximity to the electrically conductive member. In this embodiment the insulating member includes an elastomeric polymer selected from the group consisting of ethylene, polymerized with at least one xcex1-olefin, and vinyl norbornene. The insulating member also includes up to about 30, preferably up to about 25, more preferably up to about 20, most preferably up to about 15 parts per hundred parts of elastomeric polymer (pphep) of a m-ethylene copolymer.
The elastomeric polymers of various embodiments of our invention may contain in the range of from about 50-90 mole percent ethylene preferably about 65-90 mole percent, more preferably about 68-80 mole percent based on the total moles of the polymer. The elastomeric polymer contains the alpha-olefin in the range of from about 10-50 mole percent, preferably in the range of from about 10-35 mole percent, more preferably in the range of from about 20-32 mole percent. The elastomeric polymers will have a vinyl norbornene content in the range of from 0.16-5 mole percent, more preferably 0.16-1.5 mole percent, most preferably 0.16-0.4 mole percent based on the total moles of the polymer. The elastomeric polymer will also have a Mooney viscosity (ML [1+4] 125xc2x0 C. ) generally in the range of from about 10-80. The elastomeric polymer will have a Mw,GPC,LALLS/Mn,GPC,DRI(Mw/Mn) greater than about 6.
The m-ethylene copolymer will generally be a copolymer of ethylene and at least one alpha-olefin, selected from one of butene-1, 4-methyl-1-pentene, pentene-1, hexene-1, octene-l and combinations thereof Such copolymers may be generally characterized in that they will have a Mw/Mn of 3 or less, and a Composition Distribution Breadth Index (CDBI) greater than 50%. Electrical insulating and/or semi-conducting compounds using these elastomeric polymer blends with m-ethylene copolymers, may be made using fillers and other constituents well known to those of ordinary skill in the art.
To attain the same cure state as commercially available ethylene, alpha-olefin, non-conjugated diene elastomeric polymers with the diene selected for example from the group consisting of 5-ethylidene-2-norbornene, 1,4-hexadiene, 1,6 octadiene, 5-methyl-1,4 hexadiene, 3,7-dimethyl-1,6-octadiene, and the like, the elastomeric polymer/m-ethylene copolymer blends described in an embodiment of our invention require lower diene levels, at substantially equivalent curative levels.
Alternatively, at the same diene content as these other ethylene, alpha-olefin, non-conjugated diene elastomeric polymers, lower curative levels will be necessary to reach the same or a higher cure state. The ethylene, alpha-olefin, vinyl norbornene elastomeric polymers of certain embodiments of our invention have a branching index below about 0.5 The lower branching index representative of the majority component of the blend permits the extruded insulating members to have a smoother surface at higher extrusion rates and a lower die swell compared to previously available commercial materials. Owing to lower diene content, the ethylene, alpha-olefin, vinyl norbornene elastomeric polymers of certain embodiments of our invention, required to achieve the same cure state as previously available ethylene, alpha-olefin, non-conjugated diene elastomeric polymer, the compounds formulated with the elastomeric polymers of our invention generally exhibit improved heat aging performance relative to the previously available ethylene, alpha-olefin, non-conjugated diene elastomeric polymer compounds. This heat aging benefit also accrues to the blends of the present invention.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.